1. Field of the Invention
The present invention relates to antenna devices and communication terminal devices. Particularly, the present invention relates to an antenna device for use in HF band RFID tags or reader/writers, a communication terminal device including this antenna device, and a communication terminal device cover.
2. Description of the Related Art
In recent years, HF band RFID systems are introduced into communication terminal devices such as cellular phones and the like so that the communication terminal device itself may be used as a reader/writer or a RFID tag. In such cases, a RFID IC chip is mounted on a printed circuit board, and an antenna is attached on a terminal casing or disposed at available space inside the terminal casing. Thus, the RFID IC chip and the antenna are electrically connected to one another via a flexible cable or a contact pin.
On the other hand, there is a known configuration in which a feed coil connected to a RFID IC chip is mounted on a control board and allows to magnetically couple with an antenna coil, as disclosed in International Publication WO 2012/173080 and Japanese Patent No. 4325621, for example. This configuration allows high frequency signals to be transmitted from the feed coil to the antenna coil via magnetic fields. Thus, the RFID IC chip and the antenna coil may be connected without using the flexible cable or the contact pin.
International Publication WO 2012/173080 discloses a configuration in which a ferrite sheet is attached on a coil antenna (antenna coil) so as to cover the whole surface of the coil antenna at a feed coil side. Japanese Patent No. 4325621 discloses another configuration in which a magnetic sheet is attached on a loop antenna (antenna coil) so as to cover, of the surface of the loop antenna at a chip type coil (feed coil) side, a section other than a section facing the chip type coil.
In the configuration described in International Publication No. WO 2012/173080, the ferrite sheet is arranged between the feed coil and the antenna coil. Thus, in cases where the distance between the feed coil and the antenna coil becomes too large, sufficient magnetic coupling may not be obtained. Further, in the configuration described in Japanese Patent No. 4325621, the size of the magnetic sheet to be attached to the antenna coil becomes smaller. This reduces the inductance value of the antenna coil, making it difficult to obtain sufficient magnetic coupling. In other words, in the configurations of the prior art references discussed above, it is difficult to improve transmission efficiency of signals between the feed coil and the antenna coil.